In the electronics industry, electrical components such as resistors, capacitors, inductors, transistors, integrated circuits, chip carriers and the like, are typically mounted on circuit boards in one of two ways. In one way, the electronic components or modules are designed to mount to the printed circuit boards (PCBs) by means of plated through-holes in which the metal leads of the modules are spaced apart and sized to fit into corresponding plated through-holes and extend a small distance beyond the undersurface of the PCB. An alternative to the through-hole technique for mounting electronic modules on PCBs is surface mount technology (SMT) wherein the leads of electronic modules are soldered to metal pads plated on the surface of a printed wiring board. In this technique, a solder paste is applied to the metal pads and subsequently the electronic components are precisely placed on the PCB such that the coplanar leads of the module contact corresponding pads on the circuit board which are coated with a layer of solder paste. The solder paste comprises a soft solder alloy typically in a powder form and dispersed in a liquid medium conventionally containing a fluxing composition, an organic solvent and a thickening agent which provides the desired viscous or paste-like consistency to the solder formulation. The solder paste typically has sufficient adhesive strength to hold the components in position until the solder is melted. After application of the solder paste and placement of the electronic components, the entire PCB assembly is heated in a reflow oven to melt the solder in the solder paste thereby forming solder joints which permanently affix and electrically connect the electronic modules to the PCB. The assembly is then washed to remove the flux residue and tested.
A widely used technique for depositing the solder paste on the pads of the PCBs is by printing. Printing of the solder paste is usually carried out by either a screening or stenciling method. Screen printing involves the use of a screen of fine wire netting with a pattern of open and masked regions. During the screen printing operation, a moving squeegee presses the paste through the mesh openings in the open pattern regions, whereby the required areas on the PCB surface are covered with a layer of paste. Screen printing is usually carried out with the screen in an off-contact position in which the downward force of the squeegee brings the screen locally in contact with the surface of the PCB while the paste is forced through openings onto the underlying PCB surface. When the screen moves back to the off-contact position, this paste remains on the substrate by the wetting and adhesion of the paste to the surface of the PCB and the screen release or snap-off. A stainless steel gauze is a typical screen material.
Stencil printing is an alternative method of solder paste application similar to screen printing, but a stencil is used instead of a screen. A stencil is a metal foil such as of brass, stainless steel or copper, provided with openings which unlike the mesh structure of the screen are fully open and do not obstruct solder paste flow. The holes in the foil are usually made by machining, chemical etching from one or both sides of the foil or by a laser cutting technique. Both on- and off-contact modes can be applied to stencil printing. In on-contact printing the stencil and the substrate remain in contact throughout the printing stroke, and are mechanically separated after completion of the squeegee stroke. This involves the raising of the substrate so that its entire surface is in contact with the underside of the stencil throughout the printing cycle and lowering the substrate at the end of the printing stroke. The on-contact printing is widely used by volume producers as the printing can be performed both in the outward stroke in the first piece and the backward stroke on the second without smearing.
Stenciling is preferable to screening if very small areas of the solder paste are to be deposited on the PCB. The use of SMT generally offers higher circuit densities inasmuch as the spacing between leads of SMT components (the lead "pitch") can be significantly reduced from that of equivalent through-hole devices. As present technology favors the use of ever higher circuit densities and finer lead pitch, stencil quality is rapidly taking on an increasingly important role in surface mount technology. Thus, the recent advent of fine and ultra fine pitch parts as well as the use of ball grid arrays (BGAs) has brought significant new demands on stencil printing. In BGAs the leads are underneath the module. The stencil application of solder paste for components with pitches of less than 20 mil is considerably more demanding than it is for components with a 40-50 mil pitch. While it has been shown that stencil printing for lead pitches as small as 12 mil is achievable, heavy reliance is placed on exacting printability. It has been estimated that approximately 70 percent of SMT solder defects are due to solder paste printing problems.
Paste volume and viscosity are critical factors in achieving the necessary solder paste printing quality for fine, ultra fine and BGA components. Insufficient solder placed on the PCBs results in opens or shorts and is a primary cause of defects due to stencil printing. On the other hand, too much solder paste inevitably results in bridging between leads and rigid solder joints which are less compliant and more prone to cracks due to PCB/module thermal coefficient of expansion differences. Around the critical edges of the closely spaced openings in a stencil, it is not unusual to find a gap of 0.002 inch or more between the underside of the stencil and the PCB substrate. The downward pressure created by the action of the squeegee on the stencil is not enough to locally deform the stencil into intimate contact with the substrate. Accordingly, hydraulic pressure on the solder paste causes some of the paste to be extruded sideways into the gap between the stencil and the substrate. This can occur at each squeegee stroke and as solder paste progressively builds up to deposit on the lower surface of the screen around each opening, the printed edge is extended into the separate spaces between the solder pads, i.e., bridging. The printed pattern is then no longer acceptable and printing must be halted while the stencil is cleaned. Frequent cleaning of stencils is a major obstacle to high speed automated printing.
Clean stencils, accordingly, are a key factor in delivering the proper amount of solder paste to the PCB substrate. If apertures are partially or fully obstructed with dried paste or foreign material, conditions are in place for a circuit failure and open, i.e., insufficient paste. The consequences of not delivering a sufficient amount of paste to a BGA land can be particularly troublesome since it is difficult to detect the absence of paste on a BGA land even when using a transmission X-ray as a diagnostic tool. Accordingly, regular cleaning of stencils is required to prevent smearing due to solder paste getting onto the underside of the stencils and to clear obstructed openings. The finer the pitch, the more critical it is to ensure that the underside of the stencils are devoid of paste residues.
A wider range of solder pastes must be removed in stencil cleaning than in PCB defluxing. No-clean paste and in some situations rosin-based paste can be left on the PCB after reflow. However, all pastes must be removed from the stencils to achieve accurate paste placement and avoid PCB defects. No-clean, unreflowed solder pastes have a general reputation of being difficult to remove from stencil surfaces. Overall, the removal of rosin type solder paste is easier than no-clean type paste and certainly not as easy as water soluble pastes. RMA rosin paste generally requires something more than water for removal from the stencil.
The cleaner that is most widely used to clean stencils, screens and even misprints on the PCBs is isopropyl alcohol. Unfortunately, there are both environmental and safety problems associated with the use of isopropyl alcohol as a cleaning agent. Isopropyl alcohol is a volatile organic compound (VOC) and a dangerous fire risk. In addition to VOC and flammability concerns, other organic solvent and semi-aqueous cleaning systems have high biological oxygen demand (BOD) and chemical oxygen demand (COD). Although present aqueous systems can be very effective, many have high pHs in addition to containing VOCs and have relatively high BODs and CODs. Further, many current stencil cleaning agents of all types emit unpleasant odors which can bring about worker discomfort, e.g., headaches.
Stencils are cleaned by a variety of methods. The typical approach has been hand wiping. However, health risks, modern speed and efficacy requirements, as well as risk of damage to thin stencils and small apertures, have moved assemblers to automated stencil cleaning equipment. This automated equipment may operate in one or more different modes including immersion, spray-in-air and ultrasonics. Regardless of the mode of cleaning operation, proper waste management is a high priority area for all operators because of the nature of the waste involved. Various methods have been and are presently used to minimize or treat both solder paste and cleaning media wastes. One zero discharge approach utilizes evaporation of excess water as a way to perform multiple wash/rinse cycles without any waste discharge. A spray-in-air wash in enclosed equipment is followed by spray-in-air rinses, each of a very short duration. All water goes into the same reservoir. The excess volume due to the rinse water undergoes a controlled evaporation process and a complete flush is thus delayed. The type of cleaning agent most compatible with this type of system would have zero VOCs.
Accordingly, there is an increasing need for stencil cleaners which are effective, safe for workers and safe for the environment.
It is thus a primary objective of the present invention to provide a cleaning composition which is effective to remove solder paste from printing applicators such as stencils, screens and the like as well as unreflowed solder paste from printed circuit boards and is safe to use and non-hazardous.
It is another objective of the present invention to provide a cleaner which can remove unreflowed solder pastes from surfaces which is safe for workers to use and handle as well as safe for the environment and-which does not include VOCs.
It is a further objective of the present invention to provide a cleaner for removing unreflowed solder pastes which is compatible with automated cleaning equipment.
Still another objective of the present invention is to provide a safe and effective method of removing solder paste from surfaces such as stencils, screens, PCBs and the like.
These and other objects of the invention can be readily discerned from the description of the invention which is set forth below and in the appended claims.